Tectonic permeability is secondary permeability caused by plate tectonic processes. It is well-known that fractured rocks have an enhanced ability to transmit fluids and potentially to act as fluid reservoirs as well, especially if the fractures are exposed to critical stress conditions. Consequently, the density and orientation of fractures relative to the local stress field are important factors in any geological model. Usually, both the historical and present-day tectonic context of the basin control these properties, and this is why it can be so useful to evaluate the tectonic drivers of brittle deformation during hydrocarbon system analysis.
The first time I really appreciated the economic value of linking predictions of tectonic deformation to the hydrocarbon potential of a sedimentary basin was in a talk by Peter Hennings on the Suban Field of Sumatra, Indonesia (Hennings et al. 2012). He presented a multi-scale analysis of the tectonic processes governing fractures and stress conditions in the Suban Field. He presented how:
- At a regional scale, oblique relative plate motion vectors increase northwestwards across the Indo-Australia–Sundaland subduction zone causing components of strike-slip strain to increase in the upper Sundaland Plate.
- At a basin scale, partitioning of oblique strain across the Sumatran Fault, above the Indo-Australia–Sundaland subduction zone, produced dextral and compressive stresses in the Sumatran back-arc and thus in the Suban Field.
- At the local scale, how determining the orientation and relative magnitude of the maximum horizontal stress and its relation to local fractures from available well data allowed him and his team to evaluate the number of fractures at critical stress conditions.
As I remember the story, this analysis ultimately allowed his team to help plan the most economic well in the history of ConocoPhillips with a deviated path designed to intersect a maximum number of productive fractured zones.
More recently, I have had the opportunity to review the tectonic evolution of the Gulf of Mexico. In this setting, it is conspicuous how the inferred trace of the Tamaulipas–Golden Lane Fault in the western Gulf of Mexico basement (Pindell 1985) is coincident with the interpreted traces of the Faja de Oro fault system in the overlying basin fill (le Roy et al. 2008), all of which
underlie the region where some of the largest hydrocarbon resources have been recovered. It seems likely that re-activations of the basement fault may be directly related to faulting in the overlying basin and relative increases in both the tectonic permeability and hydrocarbon yield along this damaged zone.
These and other examples demonstrate the value of understanding the tectonic controls and deformation history of a prospective basin at multiple scales. The question for the hydrocarbon geologist is where else could a focus on tectonic permeability yield a competitive advantage? To assess this opportunity, a high-level workflow can be identified as follows (e.g. Hennings et al. 2012):
- Identify a basin with a classic hydrocarbon system (i.e. where source rocks, reservoir rocks, traps, and thermal maturation are givens), but where a history of deformation is evident.
- Reconstruct its tectonic evolution identifying, if possible, major faults and diffuse deformation zones.
- Reconstruct its modern tectonic setting in which boundary conditions for its active state of stress can be hypothesized.
- If possible, extend the structural and stress predictions to the third dimension using seismic and potential field data.
- If possible, evaluate the local stresses on local fractures using down-hole well imagery and logs.
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Hennings, P, P Allwardt, P Paul, C Zahm, R Reid Jr, H Alley, R Kirschner, R Lee, and E Hough (2012). Relationship between fractures, fault zones, stress, and reservoir productivity in the Suban gas field, Sumatra, Indonesia. AAPG Bulletin 96, 753–772, DOI 10.1306/08161109084.
Pindell, J (1985). Alleghenian reconstruction and subsequent evolution of the Gulf of Mexico, Bahamas, and Proto-Caribbean. Tectonics 4, 1–39, DOI 10.1029/TC004i001p00001.
Le Roy, C, C Rangin, X Le Pichon, H Nguyen Thi Ngoc, L Andreani, and M Aranda-Garcia (2008). Neogene crustal shear zone along the western Gulf of Mexico margin and its implications for gravity sliding processes. Evidences from 2D and 3D multichannel seismic data. Bulletin de la Societe Geologique de France 179, 175–193, DOI 10.2113/gssgfbull.179.2.175.